Project Summary To infect a host, bacteria must recognize niches that support growth and interact or compete with members of the microbiota to establish residence in a niche to promote disease or persistence. Campylobacter jejuni is a leading cause of bacterial diarrheal disease in the United States and throughout the world. C. jejuni is also a commensal bacterium of the intestinal tracts of many animals in the wild and agriculture, especially avian species. Sporadic cases of C. jejuni diarrheal disease are most often associated with consumption or handling of contaminated chicken meats. Thus, understanding mechanisms for how C. jejuni senses and responds to avian or human intestinal components during initial events in infection are useful for developing therapeutic strategies that may reduce the burden of C. jejuni diarrheal disease in humans and the presence of the bacterium in agriculture and meats for human consumption. We discovered that C. jejuni senses and responds to metabolites generated by the intestinal microbiota of chickens to influence expression of genes essential for commensal colonization of avian hosts, with some genes also functioning in pathogenesis of diarrheal disease in humans. We found that short-chain fatty acids (SCFAs) such as butyrate and acetate that are abundant in the lower intestinal tract, stimulated expression of many genes required for colonization of chicks. In contrast, the organic acid lactate, which is produced at higher levels in the upper intestinal tract, repressed expression of these same genes. We propose that C. jejuni senses SCFAs and organic acids produced by the resident microbiota to discriminate between different intestinal regions and identify lower intestinal niches that are ideal to support in vivo growth to establish a persistent, commensal colonization of the intestines of poultry. Since the metabolites and microbiota species that produce them are similarly spatially organized in the human intestines, we propose that C. jejuni monitors the same metabolites to identify ideal human colonic niches that support growth to establish an infection for diarrheal disease. In Aim 1, we will explore the C. jejuni SCFA- and lactate-modulated regulon and identify new colonization determinants of C. jejuni. In Aim 2, we will employ multiple approaches to identify pathways in C. jejuni involved in sensing SCFAs and lactate and mediating expression of colonization and virulence genes. In Aim 3, we will explore how in vivo manipulation of SCFAs and lactate levels or the microbiota that generate them impacts C. jejuni for commensal colonization of the avian intestines. Accomplishment of these aims will promote new insights for: 1) how C. jejuni senses and responds to in vivo metabolites and factors to recognize ideal niches for establishing infection; 2) factors required for C. jejuni colonization and virulence; 3) regulatory mechanisms for C. jejuni gene expression; 4) how manipulation of the human or avian intestinal metabolites and the microbiota impacts interactions of C. jejuni with hosts; and 5) strategies to reduce the burden of C. jejuni diarrheal disease and its presence in agriculture to create safer meat products for human consumption.